say, thrcG times as great as the radius of tlie sphere Q. Then the 

 observer receives little light from (2^ ; lie \\\\\ consiOer the level Q 

 to lie outside the solar limb. 



-0- 



Q UHM 









Fio-. 2. 



i5' E 



If, on the other hand, in a layer P the gradients are so mneh 

 steeper, that there the average I'adius of curvature of tangential 

 rays is smaller than, say, one third of the radius of the sphere P, 

 we may expect a sensible fraction of the light that P^ receives from 

 the interior to get sufficiently deviated in the region surrounding 

 /*,, so as to proceed towai-d the eaiih along the tangent P^E. The 

 observer will now consider /\ to belong to the solar disk. 



The transition from disk to surroundings will appear abrupt if 

 the minimum distance between levels like P and levels like Q be 

 less than 700 kilometers (one second of arc). This condition is com- 

 patible with a rathei' slow radial pressure gradient, because it onlj- 



dn 



I'cqu.ires that the average radius of curvature^) (Q=:n:— ) of rays 



1) "Average radius of curvature"' is here used as an abbreviated expression for : 

 "llic radius of curvalure corresponding to the average value of' thai radial com- 

 ponent of llie irregular density gradients, which is directed toward the centre of 

 the sun." 



